Fluorescent night dives, also known as fluo-diving and UV diving have been on offer at select dive centers for a number of years now and this unique style of diving is growing in popularity year on year. The first commercially available lights for viewing underwater fluorescent hit the market at the start of the century and more recently numerous manufacturers have jumped onto the fluo-band wagon, driving down costs and opening up the experience to the general diving community.

But fluo-diving is hardly new. It is thought that the first person to take ultraviolet lights into the sea was Richard Woodbridge III way back in the 1950s, who built his own underwater UV lights and took them diving in the chilly Atlantic, off the coast of Maine, USA.

Diver using Light & Motion Nightsea light. Photographs Alex Tyrrell

Some people are under the impression that fluo-diving is a little gimmicky. However everybody that I have taken out on a fluo dive has absolutely loved it, surfacing with a broad grin, raving to their friends about the dive once back on land, a common comparison being a real-life version of the Hollywood blockbuster, Avatar.

But apart from the fun factor, there is also a scientific purpose, with a number of possible applications of the use of blue light to reveal fluorescence; conservationist apply the technique for Reef Check monitoring to assess the health of coral reefs plus identify the location of coral recruits (tiny coral larvae) that would be extremely difficult using normal light. It is also used for scientific research in marine biology, as well as for the discovery and extraction of naturally occurring pigments for genetic, microbiological and medical research.

There are various theories as to why marine creatures have evolved to fluoresce. Scientific debate as to why corals even contain fluorescent proteins is ongoing. Possibilities include communication between species, for threat display behaviour, a means of camouflage, adaptation to the predominantly blue light that is found underwater for improved photosynthesis, or it could simply be that fluorescence is a by-product of another evolutionary process.

Ports coral

To slightly simplify the science behind fluorescent, certain creatures absorb electromagnetic radiation, in our case light, at one wavelength and some of this light (the amount varying from one orgasm to another) is then re-emitted at a different wavelength.

Some, but not all marine organism contain a protein, generically called the green fluorescent protein, that absorbs high energy, shorter wavelength light and then re-emits the light at the lower energy, higher wavelengths. This is different to how we normally see light that is simply reflected off an object.

This is also different to a process known as bioluminescence that you may have previously experienced on night dives, where certain creatures contain the necessary chemicals to create their own light, similar to how fire flies and glow worms can produce light in the terrestrial world. The most common type of bioluminescence witnessed by divers is when unicellular organisms called dinoflagellates are disturbed by motion, which causes them to glow as the chemical reaction inside is initiated. Other forms of marine bioluminescence are flashlight fishes, comb jellies and some deep-sea creatures, like anglerfish that have a glowing lure to attract their prey in the dark depths of the ocean.

Deep water bioluminescent jellyfish. Credit Shutterstock

Bioluminescence and fluorescence are both types of luminescence. Phosphorescence is another example of luminance that is similar to fluorescence in that it needs to be stimulated by shining light on to it, however, the former will continue to emit light after the light source shining onto it is extinguished, whereas fluorescent creatures will cease to emit light once the light shining onto them stops.

To enjoy fluorescence in the marine environment some specialised equipment is required. First of all, you need a light source at the correct wavelength that will stimulate an organism to fluoresce. This is either UV, at 400nm or less, or blue-light around 450-480 nm. The later has been proved to be more effective at stimulating a broader range of creatures to fluoresce at a greater intensity.

Cycloseris Coral

Higher quality lights will also contain an internal blue-pass filter (called an ‘excitation filter’ or ‘dichroic filter’) used for 'trimming' the spectrum of the light output to provide superior results. If you are using a blue-light source, you will also need a yellow coloured mask visor (barrier filter) as some blue-light will reflect back from whatever it is being shined upon, making the scene appear overly-blue. The mask visor strips away the excess light, leaving only the emission colours behind. Without this visor, you can still see more intense fluorescence, but the blue light will certainly overpower any subtle colours.

If you choose to use a UV light, then the yellow mask filter is not required, as UV light, also known as ‘Black Light’, is not visible to the human eye and will therefore not interfere with the fluorescent effect.

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